1. Field of the Invention
The present invention relates to a piezoelectric thin film and a method of manufacturing the same. The present invention also relates to an angular velocity sensor including a piezoelectric thin film, and a method of measuring an angular velocity by the sensor, and to a piezoelectric generating element including a piezoelectric thin film, and a method of generating electric power using the element.
2. Description of Related Art
Lead zirconate titanate (PZT: Pb(ZrXTi1-X)O3, 0<X<1) is a typical ferroelectric material capable of storing a large amount of electric charge, and used in capacitors and thin film memories. PZT exhibits pyroelectricity and piezoelectricity based on the ferroelectricity thereof. Because of not only its high piezoelectric performance but also easy adjustability of its mechanical quality factor Qm achieved by adjusting the composition or doping an element, PZT is used in a wide variety of applications such as sensors, actuators, ultrasonic motors, filter circuits, and oscillators.
On the other hand, PZT contains a large amount of lead as one of its constituent elements. In recent years, there has been a growing concern that lead leached from waste devices may cause serious damage to the ecosystem and the environment. Accordingly, there has been an international movement toward restricting the use of lead, but PZT cannot meet the requirement for this restriction. For this reason, lead-free ferroelectric materials have been in demand.
One of the lead-free ferroelectric materials that are currently under development is a perovskite-type composite oxide [(Bi0.5Na0.5)1-YBaY]TiO3 containing bismuth (Bi), sodium (Na), barium (Ba), and titanium (Ti). It has been reported that this material exhibits high piezoelectric performance of about 125 pC/N in terms of a piezoelectric constant d33, although lower than that of PZT, when the content of barium Y (=[Ba/(Bi+Na+Ba)] is 5 to 10% (see JP 4 (1992)-60073 B, and T. Takenaka et al., Japanese Journal of Applied Physics, Vol. 30, No. 9B, (1991), pp. 2236-2239).
In JP 2007-266346 A (particularly paragraphs 0034, and 0052 to 0054) and N. Scarisoreanu et al., Applied Surface Science, Vol. 252, (2006), pp. 4553-4557, attempts to produce a (Bi,Na,Ba)TiO3 film that is oriented in a specific direction have been made. It is expected that the alignment of the polarization axes in the (Bi,Na,Ba)TiO3 film with the orientation improves the ferroelectric properties of the film, such as remanent polarization and piezoelectric performance. JP 2007-266346 A describes that when a LaNiO3 (LNO) film or a SrRuO3 (SRO) film having a plane orientation of (100) is formed on an electrode and a (Bi,Na,Ba)TiO3 film is formed further thereon, the resulting (Bi,Na,Ba)TiO3 film is oriented in the (100) direction. Scarisoreanu et al., Applied Surface Science, Vol. 252, (2006), pp. 4553-4557 describe that when a (Bi,Na,Ba)TiO3 film is formed on a MgO substrate having a plane orientation of (100) by pulsed laser deposition (PLD), the resulting (Bi,Na,Ba)TiO3 film is slightly oriented in the (100) direction.
JP 10(1998)-182291 A describes that a substrate having a surface on which a buffer layer is formed controls the orientation of a Bi4Ti3O12 ferroelectric film (see particularly paragraph 0020), although it does not refer to a (Bi,Na,Ba)TiO3 film. Preferably, the buffer layer described in this publication contains all or a part of the constituent elements of the ferroelectric film to be formed thereon.